A pitching torque is created by a spinning a fan blade/propeller due to centrifugal effects. This torque occurs because blades tend to have more mass spread on one axis than on the other axis because they are relatively flat. This torque varies with the mass, the shape of the blade the square of the rpm, and the pitch of the blades. For variable pitch fans this pitching torque can add significant load onto pitching mechanisms so ways to reduce the torque can be a particularly big benefit for variable pitch fans.
One solution is to use counterweights. By distributing mass in a plane perpendicular to the cord of the blade, you create an opposing torque. This can be used to reduce the centrifugal pitching torque, or if enough mass is placed perpendicular to the chord length you can fully balance and eliminate the centrifugal pitching torque altogether.
The problem with the counterweight solution is that counterweights increase the overall weight of the fan. And in some situation, a light weight fan is necessary so as not to overload the fan drive mechanism. Therefore a goal of reducing the pitching torque with less increase of the weight of the fan was sought.